This invention relates generally to semiconductor power switching devices and, more particularly the invention relates to voltage controlled thyristor having improved switching speed and reduced drive complexity.
Until recently, the only power switching devices available in electrical power systems were bipolar transistors and thyristors. In the NPN bipolar transistor, electrons are injected from the N type emitter (cathode) across the P type base and into the N type collector (anode). In the ON state, holes flow from the base throughout the device and thus the lightly doped collector is conductivity modulated. This results in a lower voltage drop than would otherwise result. The increased charge thus stored results in a long turnoff delay.
The thyristor is a four layer diode having very high current handling capability because carriers are injected from both the cathode and anode. This is called double injection and results in better conductivity modulation than in the bipolar junction transistor where carriers are injected from only one side. The thyristor is much more tolerant of surge current than the bipolar junction transistor and virtually immune to the second breakdown effects which plague high power bipolar transistors. Standard thyristors have a relatively simple structure because they do not need the interdigitated base contact structure in order to provide base drive current, such current comes from the holes injected from the anode.
Standard thyristors cannot be turned off without interrupting the current flow. This is done either by relying on the natural reversal of current direction in an AC circuit or by providing separate and expensive self-commutating circuits. If the gate contact is highly interdigitated, as in a bipolar transistor, and of sufficiently low resistance, the device can be turned off by diverting enough current from the gate to remove the forward bias of the cathode-base junction. In this case, the device is called a gate turnoff thyristor (GTO). A GTO requires high gate currents during turnoff, typically 20-50% of the anode current, and are more expensive than comparably rated thyristors.
In the metal-oxide-silicon transistor (MOST) electrons are ejected from the cathode to anode along a field induced channel. There is no minority carry injection and storage, thus the MOST is very fast. MOST devices now dominate the market for switching power supplies and motor controls for applications below 200 volts and 100 amperes.
The insulated gate bipolar transistor (IGBT) is a hybrid MOS/bipolar combination device which provides higher current capability while at the same time retaining the attractive features of the MOST. See U.S. Pat. No. RE 33,209, for example. The IGBT is a thyristor in which the cathode and P-type base are shorted, and the gate electrode is provided to induce a channel from cathode to the N type base region. It is similar to the MOST with a P type region added to the anode. Thyristor action is discouraged by the short; however, holes will be injected from the anode to conductivity modulate the base as in a thyristor. The device has much higher current capability than the MOST; however, because the region near the front of the cathode is not conductivity modulated by virtue of the cathode-base short, the current handling capability is less than that of the GTO.
Another structure designed to realize the high current capability of the thyristor in an MOS controlled device is the MOS controlled thyristor (MCT) which uses a P channel MOST between the cathode and P base and which can turn the device off in the manner of a GTO. However, in the MCT the high turnoff current of the GTO gate flows internally through the MOST and the external circuit need only provide for charging the capacitance of the gate. A similar device is the field controlled thyristor in which the MOS transistor of the MCT is replaced by a vertical junction field effect transistor (JFET). Choice of turnoff transistor type makes little difference on device performance; however the drive circuit on JFET versions must supply high currents as in the case of the GTO.
The present invention is directed to an improved power switching device particularly applicable in high power pulse width modulation power supplies and having improved switching speed and high standoff voltage in the OFF state.